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Brooklyn College
October 1967
An Analysis of Heterochromatin in Maize Root
Tips
Marion Himes
CUNY Brooklyn College
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Recommended Citation
Himes, M. (1967). An Analysis of Heterochromatin in Maize Root Tips. The Journal of Cell Biology, 35(1), 175-181.
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AN ANALYSIS OF HETEROCHROMATIN
IN MAIZE ROOT TIPS
MARION HIMES
From the Department of Biology, Brooklyn College of The City University of New York,
Brooklyn, New York 11210
ABSTRACT
The B chromosomes of maize are condensed in appearance during interphase and are relatively inert genetically; therefore they fulfill the definition of heterochromatin. This heterochromatin was studied in root meristem cells by radioautography following administration
of tritiated thymidine and cytidine, and was found to behave in a characteristic way, i.e.
it showed asynchronous DNA synthesis and very low, if any, RNA synthesis. A cytochemical comparison of normal maize nuclei with nuclei from isogenic maize stock containing
approximately 15-20 B-chromosomes in addition to the normal complement has revealed
the following: (a) the DNA and histone contents are greater in nuclei with B chromosomes;
(b) the proportion of DNA to histone is identical with that of nuclei containing only normal
chromosomes; (c) the amount of nonhistone protein in proportion to DNA in interphase is
less in nuclei with B chromosomes than in normal nuclei. In condensed B chromosomes the
ratio of nonhistone protein to DNA is similar to that in other condensed chromatin, such
as metaphase chromosomes and degenerating nuclei. The B chromosomes appear to have
no effect on nucleolar RNA and protein. Replication of B chromosomes is precisely controlled and is comparable to that of the ordinary chromosomes not only in synthesis for
mitosis but also in formation of polyploid nuclei of root cap and protoxylem cells.
A consistent picture of heterochromatin has
emerged in the last few years (7, 25). Several
characteristics of heterochromatin have been described frequently enough to be included in a
definition. These characteristics are as follows:
(a) a condensed state in interphase; (b) genetic
inertness; (c) late, asynchronous DNA replication;
(d) low RNA synthesis in interphase. The concept
of a differential, reversible state of condensation
of a chromosome or part of a chromosome was
first suggested by Cooper (8), who implied the
control of gene activation by heterochromatization. The usefulness of this concept has been
demonstrated by recent work on the X chromosome of female mammals (17) and the paternal
chromosomes of male coccids (3, 7). The B
chromosomes of maize have been shown to possess
the first two characteristics of heterochromatin
described above (23), and the latter two characteristics will be demonstrated here. In order to
elucidate the nature of inactivated chromatin,
this study of the B chromosomes was undertaken.
The number of B chromosomes found in nature
is variable. High numbers of B chromsomes may
be obtained with inbreeding and selection, since
the disjunction of these chromsomes at meiosis is
random (23). There is little phenotypic expression
of their presence even when the number of B
chromosomes exceeds that of ordinary chromosomes (2 N = 20). In interphase nuclei with large
numbers of B chromosomes, there are regions of
dense chromatin near the nuclear periphery in
addition to the characteristic nucleolar-associated
chromatin (19). The appearance of this condensed
chromatin is similar to that of the inactive chromatin described in other organisms. In this study
175
the interphase chromatin of normal maize nuclei
is compared to that of maize nuclei containing
approximately 20 B-chromosomes. Two methods
of analysis are used: (a) cytochemical determinations of DNA, histone, and nonhistone protein,
and (b) radioautography following incorporation
of tritiated nucleic acid precursors. The present
paper describes the histone and nonhistone protein content of interphase nuclei with B chromosomes. In addition, the effect of this heterochromatin on nucleolar RNA and protein is analyzed.
Also examined is the replication of the B chromosomes in polyploid cells.
MATERIALS AND METHODS
Maize seeds (Zea mays) from a self-fertilized plant
with 14 B-chromosomes and seeds from an isogenic
stock without B chromosomes were used. Seeds from
a stock with 25 B-chromosomes were also used.'
After germination on wet filter paper until the roots
were 4-8 mm in length, the root tips were removed
and fixed either in 10% neutral formalin or by freezesubstitution (30), or they were placed in isotope for
suitable periods of time and then removed and fixed
similarly. The root tips were embedded in paraffin
and sectioned at 4 or 20 /p. Sections of comparably
treated normal root tips and B-chromosome-containing root tips were mounted together on the same
slide for exact comparison.
Cytochemical Analysis
The Feulgen method of Stowell (27) was used for
DNA determinations. Trichloroacetic acid was substituted for HCI in the hydrolysis for DNA-histone
ratios. After photometric measurements of DNA
content were made, the same slide was stained by
the Alfert-Geschwind method for histone (2), and
the same cells were remeasured for histone content.
The method for estimation of nonhistone protein
by means of acid dyes at low pH (1.5-2.5) depends
on the following rationale. When nucleic acids are
removed from freeze-substituted material by hot
trichloroacetic acid, the binding of acid dyes is increased. When the hot trichloroacetic acid is followed
by dilute HCl extraction of histones, the amount of
acid dye bound is approximately equal to the amount
bound in the root tips that had neither the nucleic
acids nor histone removed. These findings suggest
that acid-dye binding by the amino groups of histones is suppressed by the presence of nucleic acids
under these conditons of fixation.
The author wishes to express appreciation to
Professor M. M. Rhoades and Professor L. F. Randolph for providing the material used.
176
THE JOURNAL OF CELL BIOLOGY
Fast green was used, following nucleic acid removal, for estimation of nucleolar total protein, and
naphthol yellow S was used following the Feulgen
reaction, according to Deitch (9), to determine
DNA:nonhistone protein ratios. Azure B was used
for nucleolar RNA determinations according to
Flax and Himes (10).
Photometric determinations of the amounts of
dye binding were made with an apparatus similar to
that described by Pollister and Moses (22). The light
source was a tungsten 6 v ribbon filament stabilized
by a storage battery; a Bausch and Lomb grating
monochrometer was employed, with entrance and
exit slits adjusted to 0.48 mm; a 16 mm objective
served as a condenser; and a Zeiss 1.4 NA apochromat 90X oil-immersion objective was used. The
single wavelength method was used for measuring
light through thin sections of homogeneously stained
regions of chromatin or nucleoli. The two wavelength method (21) was used for determining
amounts of Feulgen stain and alkaline fast-green
stain; the wavelengths chosen were 570 and 510
mu for the former, and 610 and 572 mu for the
latter. For DNA determinations 20-,u thick sections
were used; for DNA measurements followed by
histone staining and measurements of dye bound,
4-, thick sections were used to avoid any error in
photometry owing to slight cytoplasmic staining.
Precautions were taken to insure that the area measured of each nuclear section was precisely the same
for the two determinations. The Feulgen-naphthol
yellow S stain was measured through a small homogeneous area of chromatin at 570 and 435 mu
where the absorption curves of the two dyes do not
overlap.
Radioautography
Tritiated thymidine (specific activity, 6.25 c/
mmole) was diluted to 2-4 c/ml in water; tritiated
cytidine (specific activity, 1.21 c/mmole) was used
at concentrations of 2-10 c/ml; both were supplied
to root tips for periods of 15 min-2 hr. Kodak AR 10
stripping film (Eastman Kodak Co., Rochester,
N.Y.) was applied to mounted sections, following
Feulgen staining, for thymidine-incorporation studies, and to unstained sections for cytidine-incorporation studies. RNase controls were used in conjunction
with the latter. The films were exposed for 2-4 wk,
developed, and stained with azure B (3 mg/ml, 30
sec).
RESULTS
DNA and Protein Composition of
B Chromosomes
Table I indicates the relative amounts of DNA
in the diploid and higher classes of maize nuclei.
VOLUME 35, 1967
Although variations in DNA content among
different strains of Zea mays are known (Swift,
reference 28), the difference in the diploid amount
of DNA in nuclei of the isogenic stocks studied
here can be due to only the B chromosomes. No
attempt was made to count the number of B
chromosomes in each root tip. The elimination of
roots with low numbers of B chromosomes was
achieved by restricting the studies to root tips with
large amounts of peripheral chromatin in interphase nuclei and with metaphase plates the
diameter of which was almost double that of
metaphase plates observed in normal maize.
Variation in DNA content per nucleus among
roots with different numbers of B chromosomes
was expected. In order to avoid variations owing
to DNA changes during the S period, only nuclei
in telophase and metaphase were chosen. The
DNA variation is similar in normal nuclei and Bchromosome-containing nuclei; this indicates
that the B chromosomes behave exactly like
normal ones with regard to the precise regulation
of DNA synthesis and anaphase separation. Also,
the amounts of DNA in nuclei of root cap and
protoxylem cells indicate as exact a duplication of
DNA in B chromosomes as in normal ones.
Table II a shows the relative amounts of staining by the Feulgen reaction and by alkaline fast
TABLE I
Relative Amounts of DNA (Feulgen Reaction)
Nuclear class
Normal maize
B-chromosome-containing
maize
Protoxylem nuclei
Meristem telophase or
metaphase/2
2c
3.41 - 0.08
0.10
4.95
4.25 4- 0.08
6.20 - 0.11
(18)
(12)
(10)
(10)
4c
8c
7.4 (4)
9.9 (4)
13.1 (5)
20.1 (3)
Root cap nuclei
4c
8c
6.7 (5)
9.6 (4)
12.1 (3)
19.8 (4)
In all cases, the mean relative amount of Feulgen dye is given - the standard error, with the number of
different nuclei measured in parentheses.
TABLE II a
DNA-Histone Ratios (Feulgen-FastGreen, pH 8.1)
Normal maize, interphase
Normal maize, metaphase
B-chromosome-containing maize,
interphase
Relative amount of
Feulgen dye
Relative amount of
fast green dye
Ratio
4.5 - 0.2*
4.8 F- 0.5
4.4 - 0.3
5.4 - 0.1*
5.6 4- 0.3
5.4 4- 0.3
0.83
0.87
0.82
* Mean - standard error
TABLE IIb
DNA-Nonhistone Protein Ratios (Feulgen-Naphthol Yellow S, pH 2.0)
Meristem
Interphase
Normal maize
B-chromosome-containing maize
B-chromosome-containing maize
0.46 4- 0.01*
0.62 -- 0.04
0.70 4- 0.05
Metaphase
0.86
0.81
0.79
- 0.03*
- 0.01
- 0.03
Root cap
interphase
1.2*
1.7
0.9
* Mean ratios of extinction of dye at 570 mu (Feulgen) to extinction of dye at 435 me
(naphthol yellow S) - standard error
MArON HIMES
Heterochromatinin Maize Root Tips
177
green measured successively in the same nuclei. It
can be seen that the ratio is the same in normal
nuclei and in nuclei containing B chromosomes.
Since many interphase cells are in the process of
replication, it appears that DNA and histone are
synthesized concurrently in the B chromosomes,
as has been shown for normal chromosomes by
Bloch and Godman (5).
When low pH acid-dye binding is compared to
Feulgen stain in the same nucleus, it is apparent
that there are significant differences in the ratios
obtained for dispersed chromatin as compared to
condensed chromatin (Table 11 b). The amount of
dye bound at low pH, an indication of nonhistone
protein, is greater in dispersed chromatin than in
condensed chromatin as shown by the low ratio of
Feulgen stain to naphthol yellow S stain (0.46).
The condensed chromatin contains less nonhistone
protein and shows higher ratios, i.e. 0.62-0.70 in
nuclei with B chromosomes, 0.81 in metaphase
chromosomes, and up to 1.7 in nuclei of root cap
cells.
Nucleolar RNA and Protein
The effect of B chromosomes on the nucleolus
is indicated in Table III, which shows comparisons
of concentrations of azure B and fast green (low
pH) bound to RNA and total protein, respectively.
The nucleolar volumes are similar in the protoxylem cells in normal and isogenic B-chromosomecontaining maize, and therefore the amounts of
RNA and protein are the same. Nucleoli of
meristem cells were not studied photometrically,
but their diameters were measured to compare
ranges of nucleolar volume throughout the cell
cycle. There was a complete overlap of ranges,
between 2.5 and 3.5 A diameter for nucleoli in the
normal and the B-chromosome-containing stocks.
When material from a different source was
examined, the nucleolar volume was significantly
greater in nuclei with B chromosomes, but, since
isogenic material with only the normal complement of chromosomes was not available, no
further studies were carried out. It is possible that
extra nucleolar-organizing chromatin was present,
having been translocated either to a normal or a
B chromosome.
That incorporation of tritiated cytidine into
nucleoli of the isogenic stocks shows that the extra
B-chromosomes have no measurable effect on
RNA synthesis agrees well with the photometric
determinations of amounts of RNA and protein.
Fig. I shows a comparison of radioautographs of
(a) a section through the meristem of a normal
root tip and (b) a section through an isogenic, Bchromosome-containing root tip. The nucleoli
that are cut at the surface show that the nucleolar
volume and the number of grains in both sections
are similar. This is also shown by grain counts
after different times in the isotope (Table III).
Chromosomal DNA and RNA Synthesis
The incorporation of tritiated thymidine into
meristem nuclei, followed by radioautography,
indicates that the S period of the B chromosomes
does not overlap to any considerable extent the S
period of euchromatin. The peripheral location of
the B chromosomes in interphase makes it possible
TABLE III
Nucleolar RNA and Protein Content and Cytidine Incorporation
Nucleolar RNA and protein content
Diameter ()
Azure B concentrationS
Fast green concentrations (pH 2.5, after
removal of RNA)
Nucleolar RNA synthesis (grain counts per
nucleolus after cytidine incorporation)
15 min
30 min
1 hr
THE JOURNAL OF CELL BIOLOGY
B-chromosome-containing
maize
5.4 4- 0.3*
1.40 4- 0.03
0.79 4- 0.02
5.1 4- 0.4*
1.32 4- 0.03
0.84 = 0.02
1.3
7.4
17.1
* Mean - standard error
$Extinction in arbitrary units
178
Normal maize
VOLUME 35, 1967
4- 0.3*
4- 0.4
4- 0.3
1.1
7.9
15.0
4- 0.5*
+= 0.3
4- 0.4
FIGURE 1
Radioautograph of maize meristem following 30 min incorporation of tritiated cytidine.
Fig. 1 a, maize with normal chromosome complement. Fig. 1 b, maize with approximately 15-20 B chromosomes. Note that labeling in nucleolar and nonnucleolar nuclear (chromatin) regions is similar in a and b.
FIGURE 2
Radioautograph of maize with B chromosomes following 1 hr incorporation of tritiated thymidine. Note that nucleus at lower right has heavy
label, perhaps exclusively over euchromnatin. Nucleus
at upper right shows label only over the heterochromatin.
to identify them in radioautographs. In many cases
a heavy label appeared over all the chromatin
except the peripheral clumps, while only rarely
were the grains located exclusively over the
peripheral regions, as in Fig. 2. This appearance is
similar to that in other examples in which late
replication of DNA in heterochromatin has been
established (14). The infrequency of the appearance of such cells (about one in a thousand)
compared to the frequency of appearance of
labeled euchromatin and unlabeled heterochromatin indicates that the relative time spent in
DNA synthesis is very much greater for euchromatin than for heterochromatin.
Chromosomal RNA synthesis was determined
by counting grains developed over nonnucleolar
areas of the nucleus after tritiated cytidine incorporation. The data presented in Table IV show no
significant difference between grains over the
nuclei with only the normal complement of chromosomes and those over the nuclei with additional
B chromosomes. Since the nuclear volume is
greater in nuclei with B chromosomes, the number
of grains per unit area is less in B-chromosomecontaining nuclei than in normal nuclei. It can be
concluded, therefore, that the B chromosomes,
which add at least DNA and histone to the interphase nucleus and increase the nuclear volume,
do not contribute to the synthesis of chromosomal RNA. This agrees well with the idea that
condensed chromatin is inactive in messengerRNA synthesis (3).
DISCUSSION
The contribution of B chromosomes to interphase
maize nuclei can be listed as follows: (a) the DNA
and histone content are increased to different
amounts in roots with different numbers of B
chromosomes, as expected; (b) DNA and histone
are maintained in the same proportion; (c) the
increase in nonhistone protein is relatively low
compared to the amounts present in euchromatin;
(d) nucleolar RNA and protein remain unchanged;
(e) there is little or no increase in chromosomal
RNA synthesis.
The possibility that heterochromatin DNA and
histone do not replicate in step with euchromatin
DNA and histone during the multiple replications
MARION HIMES
Heterochromatin in Maize Root Tips
179
TABLE IV
Cytidine Incorporation in Chromnatin
Grain counts per nucleus
(over nonnucleolar areas)
30 min exposure
Normal maize
B-chromosome-containing maize
forming polyploid or polytene nuclei has been
suggested by several workers. In the giant nurse
cells of Sciara coprophila the L chromosomes that
are present only in the germ line and are heterochromatic do not appear to replicate, while the
other chromomes reach a DNA level which is
about 1024 times the diploid amount of DNA
(Himes and Crouse, reference 12). Also, on the
basis of photometric determinations, Rudkin (26)
has indicated that the heterochromatin of the Y
chromosomes in Drosophila salivary gland cells
does not replicate along with the other chromosomes. In addition, Nur (20), on the basis of
purely morphological evidence, has stated that the
heterochromatic paternal set of chromosomes does
not replicate along with the euchromatic maternal set in the giant oenocytes of male coccids. In
contrast to these three cases, the B chromosomes
of maize appear to replicate in a regular fashion
in the polyploid nuclei of the root cap and protoxylem cells. Although the B chromosomes are
similar to the Y chromosome of Drosophila, the L
chromosomes of Sciara, and the paternal set of
chromosomes of male coccids with respect to late
replication of DNA and low RNA synthesis, there
appears to be a difference regarding the question
of multiple replications. This may be correlated
with the fact that the B chromosomes are not
constant components maintained in the species as
are the other chromosomes. It seems possible that
the control of replication of heterochromatin has
evolved in some organisms as an economy that
prevents unnecessary syntheses, perhaps paralleling the evolution of chromosome diminution,
which also eliminates the replication of unnecessary DNA in the somatic line. No such mechanism
may have evolved in maize, in which the B
chromosomes vary in number in nature and are
usually absent
The fact that B chromosomes do not contribute
to the amount of RNA and protein of the nucleolus
is in contrast to the findings of Lin (15), who
180
TEim JOURNAL OF CELL BIOLOGY
3.8
3.4
±
0.4
0.5
1 hr exposure
8.5
0.7
8.8 t 0.6
found a small increase in nucleolar RNA and
protein in microsporocytes of maize containing B
chromosomes. The reason for assuming that
heterochromatin might affect the nucleolar RNA
and protein content is that typically the nucleolarorganizing region of the chromosomes is heterochromatin. The significance of this has yet to be
explained. There is excellent evidence that the
DNA coding for ribosomes is present at this
location (24, 29) and that repeated segments of
nucleolar organizer material are located at this
site (18). Perhaps the heterochromatic condition
of the nucleolar-associated DNA represents inactive sites that are repeated parts of the genome
capable of coding for ribosomes. The heterochromatin of the B chromosomes may be qualitatively
different, and so may not affect the nucleolus.
The same proportion of histone to DNA has
been found in condensed chromatin and dispersed
chromatin of maize in contrast to the finding of
differences that have been reported for chromatin
in coccids (4). The absence of increased histone in
heterochromatin offers no support for the idea
that histone regulates the amount of DNAdependent RNA synthesis. The method used for
determining histones, however, does not eliminate
the possibility that histones could differ greatly
qualitatively in the two conditions of chromatin
and still maintain the same proportion of dyebinding sites. Other cytological methods have
been used to distinguish histones in condensed
chromatin and dispersed chromatin by Littau et
al. (16) and by Himes (11) who obtained different
results. At the present time, while the in vitro
repression of DNA activity by histones is clear (6),
any demonstration of histone involvement in
heterochromatin inactivity is lacking.
The relative amount of nonhistone protein in
the B chromosomes was found to be less than that
of nonhistone protein in euchromatin in interphase
maize nuclei. This characteristic of typical heterochromatin is shared by other condensed chromatin,
VOLUME 35, 1967
such as metaphase chromosomes. In two additional
cases, the appearance of condensed chromatin has
been characterized by loss of nonhistone protein:
during pycnosis (1) and during spermatogenesis
(13). Another characteristic of both heterochromatin and other condensed chromatin is the low
RNA synthesis. The two phenomena are probably
interrelated. The possibility that the nonhistone
protein present in dispersed chromatin may be
involved in strand separation of the DNA molecule
that may be necessary for RNA synthesis is open
to speculation.
Received for publication 31 March 1967; revision accepted
13 June 1967.
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Heterochromatinin Maize Root Tips
181